Direct Expression of Polypeptides
Many polypeptides of pharmaceutical importance are prepared in prokaryotic cells such as bacteria using recombinant DNA techniques. The DNA encoding the polypeptide (often obtained from human tissues) can be inserted into the "host" cell where it is expressed, (i.e. translated) from DNA to polypeptide in the cytoplasm. The polypeptide is then purified from the host cells. This procedure is often referred to as "direct" expression of a polypeptide.
While direct expression of a polypeptide is often the most convenient way to manufacture the polypeptide, there can be problems associated with this method.
For example, as the polypeptide is synthesized in the host cell and begins to accumulate, the polypeptide may become toxic to the host cell and kill it. In addition, intracellular proteases may rapidly degrade the polypeptide molecules as they are synthesized. Further, as the intracellular concentration of the polypeptide increases, the host cell machinery may cease manufacturing via a "feedback mechanism" in the host cell directing termination of polypeptide synthesis. One other possibility is that polypeptides that remain in the cytoplasm after synthesis may be subjected to post-translational modification such as acetylation.